Applied Microbiology and Biotechnology (2021) 105:3533–3557 https://doi.org/10.1007/s00253-021-11289-9 MINI-REVIEW Sporulation in solventogenic and acetogenic clostridia Mamou Diallo1,2 & Servé W. M. Kengen2 & Ana M. López-Contreras1 Received: 21 December 2020 /Revised: 3 April 2021 /Accepted: 7 April 2021 / Published online: 26 April 2021 # The Author(s) 2021 Abstract The Clostridium genus harbors compelling organisms for biotechnological production processes; while acetogenic clostridia can fix C1-compounds to produce acetate and ethanol, solventogenic clostridia can utilize a wide range of carbon sources to produce commercially valuable carboxylic acids, alcohols, and ketones by fermentation. Despite their potential, the conversion by these bacteria of carbohydrates or C1 compounds to alcohols is not cost-effective enough to result in economically viable processes. Engineering solventogenic clostridia by impairing sporulation is one of the investigated approaches to improve solvent produc- tivity. Sporulation is a cell differentiation process triggered in bacteria in response to exposure to environmental stressors. The generated spores are metabolically inactive but resistant to harsh conditions (UV, chemicals, heat, oxygen). In Firmicutes, sporulation has been mainly studied in bacilli and pathogenic clostridia, and our knowledge of sporulation in solvent- producing or acetogenic clostridia is limited. Still, sporulation is an integral part of the cellular physiology of clostridia; thus, understanding the regulation of sporulation and its connection to solvent production may give clues to improve the performance of solventogenic clostridia. This review aims to provide an overview of the triggers, characteristics, and regulatory mechanism of sporulation in solventogenic clostridia. Those are further compared to the current knowledge on sporulation in the industrially relevant acetogenic clostridia. Finally, the potential applications of spores for process improvement are discussed. Key Points • The regulatory network governing sporulation initiation varies in solventogenic clostridia. • Media composition and cell density are the main triggers of sporulation. • Spores can be used to improve the fermentation process. Keywords Solventogenic clostridia . Sporulation . Quorum sensing . Sigma factors . ABE production . Acetogens Introduction towards more sustainable economies, and countries world- wide are promoting the renewable energy sector. While sub- As a growing part of the world’s population is getting access stitutions to fossil energy generation processes such as hydro- to affordable energy, the global energy demand is increasing thermal, geothermal, solar, or wind energy are promoted, few drastically. The energy consumed mainly originates from fos- alternatives to oil for freight, aviation, or the petrochemical sil resources, resulting in an acceleration of the depletion of sector are cost-effective. “Advanced biofuels,” defined by natural resources and increased greenhouse gas (GHG) emis- the International Energy Agency (IEA) as liquid or gaseous sions. To inverse this trend, our societies are transitioning fuels derived from lignocellulosic (second-generation biofuel) or algal biomass (third-generation biofuel), are among the most promising substitutes to oil. These feedstocks have a * Mamou Diallo relatively diverse composition and are available as complex [email protected] polymeric structures (lignocellulose) that require dedicated * Ana M. López-Contreras enzymes to release the fermentable sugars. Some bacterial [email protected] species from the Clostridium genus can hydrolyze these poly- 1 Wageningen Food and Biobased Research, mers, and ferment the carbohydrates to produce solvents. Wageningen, The Netherlands Clostridia are anaerobic and spore-forming Gram-positive 2 Laboratory of Microbiology, Wageningen University & Research, bacteria, and the entire Clostridium genus currently comprises Wageningen, The Netherlands over 270 species (https://www.bacterio.net/), including 3534 Appl Microbiol Biotechnol (2021) 105:3533–3557 pathogenic, probiotic, thermophilic, and benign soil bacteria. situ solvent removal techniques to prevent toxicity (Annous and However, the Clostridium genus proposed by Prazmowski in Blaschek 1991; Qureshi and Blaschek 2001;Wangetal.2019; 1880 (Prazmowski 1880) is not a monophyletic group Xu et al. 2015). It appears that both solvent toxicity and sporu- (Collins et al. 1994;Jones2001; Yutin and Galperin 2013), lation prevent the production of higher solvent titers (Cheng et al. and only a subset of the 16S rRNA tree (cluster I), Clostridium 2019; Papoutsakis 2008). During the fermentation, bacteria are sensu stricto is currently recognized as the genus’srepresen- grown under stringent conditions, which are necessary for sol- tative (Cruz-Morales et al. 2019;GuptaandGao2009; vent production yet harsh for the cells. As a defense mechanism, Lawson and Rainey 2016). Several non-pathogenic clostridia the bacteria differentiate into highly resistant cells called endo- have been studied for the production of advanced fuels and spores (henceforth designated as spores) while producing sol- other biochemicals (Tracy et al. 2012). These species are com- vents. Spores are metabolically inactive, and their formation monly divided into acid-producing, solvent-producing, cellu- not only requires metabolic energy but also impairs solvent pro- lolytic, and acetogenic species (Dürre 2005). These bacteria ductivity (Tracy et al. 2012). Several efforts were made to engi- convert simple and complex carbon sources, from C1 com- neer asporogenous solvent-producing strains, to prevent these pounds to cellulose, into a diverse range of metabolites, rang- undesirable effects (Al-Hinai et al. 2014;Bietal.2011; ing from carboxylic acids such as acetate or butyrate to sol- Scotcher and Bennett 2005). The regulation of sporulation and vents like butanol and propanol. Ten Clostridium species are solventogenesis appears to be coupled, but the underlying regu- known to be solventogenic (Poehlein et al. 2017), see Table 1, latory networks remain unclear (Patakova et al. 2013). Due to the with C. acetobutylicum, C. beijerinckii, C. pasteurianum, C. lack of efficient engineering tools for Clostridium, studies on the saccharobutylicum,andC. saccharoperbutylacetonicum be- regulation of sporulation have been scarce. The recent develop- ing the most studied. These species have been used industri- ment of markerless tools for clostridia (Atmadjaja et al. 2019; ally during the twentieth century for acetone production Cañadas et al. 2019;Dialloetal.2020a; Huang et al. 2019; through the ABE fermentation process (Berezina et al. 2012; Joseph et al. 2018;Lietal.2019;Seysetal.2020;Wasels Jones and Woods 1986;Jones2001; Sauer 2016), but other et al. 2020;Zhaoetal.2019) has made the genetic engineering solventogenic strains with high solvent productivity were iso- of clostridia much more attainable. As a result, the number of lated recently (Xin et al. 2018). engineered clostridia increased substantially and together with Despite many efforts, bioprocesses relying on these bacteria the rise of omics studies, the current knowledge on sporulation are not cost-effective due to high feedstock costs, expensive pre- in solventogenic clostridia has expanded considerably. treatment of the feedstock, poor substrate use, and low solvent productivity (Green 2011; Tashiro et al. 2013). These issues need to be tackled to enable competitive biofuel prices. According to Sporulation regulation in solventogenic estimates, the product titer in a bioprocess aiming for the biofuel clostridia market needs to reach at least 50 g L-1, and the productivity should amount to 3 g L-1 h-1 to be commercially viable (Vees In response to changes in the environment, some bacteria pro- et al. 2020). While in standard batch conditions, solvent produc- duce spores to survive under unfavorable conditions. tivity has been reported to reach 10 g L-1 h-1 with clostridial Depending on the formation mechanism and the structure, fermentation, the clostridial maximum butanol titer in batch fer- different spore types can be found in the environment (Paul mentation could not exceed 25 g L-1 without the application of in et al. 2019). The spores, formed by Firmicutes, called Table 1 Solventogenic clostridia available in international strain collections and main fermentation products Specie Products Reference C. acetobutylicum Butanol, ethanol, acetone, acetate, butyrate, propanediol (Forsberg 1987;JonesandWoods1986; Keis et al. 2001; Shaheen et al. 2000) C. beijerinckii Butanol, ethanol, acetone, acetate, butyrate, (Diallo et al. 2018; Forsberg 1987;Keisetal.2001; Mate de 2,3-butanediol, 2- propanol, propionate, n-propanol, Gerando et al. 2018;Raedtsetal.2014;Sedlaretal.2021; propanediol, isopropanol Shaheen et al. 2000) C. pasteurianum Butanol, ethanol, acetone, acetate, butyrate (Gallazzi et al. 2015;Malaviyaetal.2012; Xin et al. 2016) C. saccharobutylicum Butanol, ethanol, acetone, acetate, butyrate (Jones and Woods 1986;Keisetal.2001; Shaheen et al. 2000) C. saccharoper-butylacetonicum Butanol, ethanol, acetone, acetate, butyrate (Jones and Woods 1986;Keisetal.2001; Shaheen et al. 2000) C. puniceum Butanol, acetone, acetate, butyrate (Berezina et al. 2012;Lundetal.1981) C. aurantibutyricum Butanol, ethanol, acetone, acetate, butyrate, isopropanol (George et al. 1983; Jones and Woods 1986) C. felsineum Butanol, ethanol, acetone, acetate, butyrate (Avrova et al.